An overmassive Dark Halo around an Ultra-diffuse Galaxy in the Virgo Cluster

TL;DR

This study measures the total mass of a Virgo cluster ultra-diffuse galaxy using its globular cluster system, revealing an unexpectedly massive dark matter halo and highlighting the stochastic nature of galaxy formation in dense environments.

Contribution

First dynamical mass measurement of a UDG in Virgo using globular clusters, showing an overmassive dark halo and proposing GC counting as an effective mass estimation method.

Findings

UDG VCC 1287 has a dark matter halo mass of ~8x10^{10} M_sun.

The galaxy's mass-to-light ratio is extremely high at ~106.

It is an outlier in the stellar mass-halo mass relation, indicating stochastic formation processes.

Abstract

Ultra diffuse galaxies (UDGs) have the sizes of giant galaxies but the luminosities of dwarfs. A key to understanding their origins comes from their total masses, but their low surface brightnesses ($\mu(V) \geq$ 25.0) generally prohibit dynamical studies. Here we report the first such measurements for a UDG (VCC~1287 in the Virgo cluster), based on its globular cluster system dynamics and size. From 7 GCs we measure a mean systemic velocity $v_{\rm sys}$ = 1071$^{+14}_{-15}$ km/s, thereby confirming a Virgo-cluster association. We measure a velocity dispersion of 33$^{+16}_{-10}$ km/s within 8.1 kpc, corresponding to an enclosed mass of $(4.5 \pm 2.8)\times10^{9}$ $M_{\odot}$ and a $g$-band mass-to-light ratio of $(M/L)_g = 106^{+126}_{-54}$. From the cumulative mass curve, along with the GC numbers, we estimate a virial mass of $\sim8\times10^{10}$ $M_{\odot}$, yielding a dark-to-stellar mass fraction of $\sim3000$. We show that this UDG is an outlier in $M_{\rm star} - M_{\rm halo}$ relations, suggesting extreme stochasticity in relatively massive star-forming halos in clusters. Finally, we discuss how counting GCs offers an efficient route to determining virial masses for UDGs.